In telephone systems, channel circuits serve as an interface between communication lines connected to customer equipment and the telephone switching network. In some systems, line concentrators are advantageously employed to allow a number of channel circuits to serve a greater number of lines while still maintaining a completely acceptable level of service. Because of advances in the technology of high-voltage semiconductor devices, such concentrators can now be implemented using semiconductor crosspoints. Although semiconductor concentrators represent a substantial advance in telephony from a number of standpoints, the battery feed provided from channel circuits through a semiconductor concentrator must be capable of floating with respect to ground. An example of a floating battery feed circuit is disclosed in U.S. Pat. No. 4,499,532 issued to R. C. Hudson et al. on Feb. 12, 1985. Because battery feed circuits employed in the past have typically provided a ground reference, many special customer circuits have been designed that rely on the ground reference for their proper operation. Although the number of these special circuits served by switching systems employing a floating battery feed is small compared, for example, to the total number of lines, the cost of replacing or providing customized interfaces for the special circuits represents a significant problem for switching system purchasers.
In view of the foregoing, a recognized problem in the art is the incompatibility of telephone switching equipment relying on a floating battery feed with special customer equipment that requires a ground reference for its proper operation.